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Sustainable Water Resources
Hydrological Cycle The diagram below illustrates components of the hydrologic cycle in a representative watershed (basin). The level of detail given of components of the hydrologic cycle varies with the purpose and scope of the particular analysis. For example, the ground-water discharge term (GWdisch) shown in the figure could be further divided to include discharges to streams, discharges to wetlands, or evapotranspiration losses directly from the water table. For pages on your watershed please use the following definitions to describe the various water flows. Water Budget Approach to Sustainable Management of Water Resources. The Water-Budget Equation The water-budget equation is simple, universal, and adaptable because it relies on few assumptions on mechanisms of water movement and storage. A basic water budget for a small watershed can be expressed as: (A1) P + Qin = ET + DeltaS + Qout where P is precipitation, Qin is water flow into the watershed, ET is evapotranspiration (the sum of evaporation from soils, surface-water bodies, and plants), DeltaS is change in water storage, and Qout is water flow out of the watershed. The elements in equation A1 and in all other water-budget equations are referred to as components in this report. Water-budget equations can be written in terms of volumes (for a fixed time interval), fluxes (volume per time, such as cubic meters per day or acre-feet per year), or flux densities (volume per unit area of land surface per time, such as millimeters per day). Typically, water budgets are tabulated in spreadsheets or tables such as that shown in table A–1, which contains monthly and yearly data for Seabrook, New Jersey, from Thornthwaite and Mather (1955). With the approach used by those authors, it is assumed that Qin is zero and Qout is equal to runoff. Equation A1 can be refined and customized depending on the goals and scales of a particular study. Precipitation can be written as the sum of rain, snow, hail, rime, hoarfrost, fog drip, and irrigation. Water flow into or out of the site could be surface or subsurface flow resulting from both natural and human-related causes. Evapotranspiration could be differentiated into evaporation and plant transpiration. Further refinement could be based on the source of the water that is evapotranspired. Evaporation can occur from open water, bare soil, or snowpack (sublimation); plants can extract ground water or water from the unsaturated zone. Such refinements must be balanced with available measurement techniques, which often are not designed, or lack sufficient resolution, to distinguish among subcomponents. Most methods for measuring evapotranspiration, for example, quantify the flux of water from the land/vegetation surface to the atmosphere and do not distinguish between different water sources. Fashioning a viable water-budget approach for estimating evapotranspiration or other water-budget components requires analysis of available measurement techniques. Water storage occurs within all three compartments of the hydrologic cycle. The amount of water stored in the atmosphere is small compared to that on land surface and in the subsurface. Surface water is stored in rivers, ponds, wetlands, reservoirs, icepacks, and snowpacks. Subsurface storage can be categorized into various subaccounting units, such as the root zone, the unsaturated zone as a whole, the saturated zone, or different geologic units. An expanded form, but certainly not an exhaustive refinement, of the water budget appropriate for many hydrologic studies can be written as (Scanlon and others, 2002): (A2) P + Qswin + Qgwin = ETsw + ETgw + ETuz + DeltaSsw + DeltaSsnow + DeltaSuz + DeltaSgw + Qgwout + RO + Qbf where the superscripts refer to surface water (sw), ground water (gw), unsaturated zone (uz); RO is surface runoff; Qgwout refers to both ground-water flow out of the site and any withdrawal by pumping; and Qbf is base flow (ground-water discharge to streams). It is unlikely that all elements in equation A2 will be of importance at any one site; some will be of negligible magnitude and can be ignored. Indeed, when selecting an accounting unit for developing a water budget, judicious selection of boundaries can greatly facilitate the accounting process. Consider, for example, a small watershed and associated shallow ground-water system. Watershed boundaries are well defined: there is no surface flow in, and surface flow out occurs only in a stream channel, where discharge can be readily measured. If watershed boundaries correspond to ground-water divides, there is also no subsurface inflow. Suppose all ground water that is not lost to ET eventually discharges to the stream; an appropriate water budget for the watershed could be stated as: (A3) P = ET + DeltaS + RO + Qbf If the annual change in storage is small, evapotranspiration can be estimated as the difference between precipitation and streamflow out of the watershed. References * Water Budgets: Foundations for Effective Water-Resources and Environmental Management Knowledgebase Pilot Category:Water Category:Resource Use